The process by which mRNA is produced from DNA is called transcription. Reverse transcription occurs in HIV. The length of DNA that an RNA transcribes is called a transcription unit. Transcription unit consists of promoter, start point and termination point. Transcription is a very fast process. It takes one second to transcribe an mRNA of 1000 nucleotides from a gene in E. coli bacteria. In actual cells, an mRNA is transcribed from a gene and a protein is translated from it. This is called monocistronic transcript. Multiple proteins are translated to transcribe a recipe in progenitor cells. This is called a polycistronic transcript.
Essential components of transcription
1. Requires mRNA template.
2. Requires RNA polymerase enzyme.
3. Requires free ribonucleotide triphosphate (ATP, GTP, TTP, UTP).
4. Chemical energy that breaks down triphosphate to produce ATP.
5. Some associate proteins are required.
Process of Transcription
Transcription is a complex process. The steps in this process are – transcription initiation, mRNA amplification, mRNA termination and mRNA termination.
1. Initiation
(i) Transcription takes place inside the cell nucleus. DNA is inside the nucleus. DNA molecules contain genes. Every gene has a promoter. The promoter sequence is 5-TATAWAWA-3. In real cells it is called TATA box or Goldberg-Hogness box. In progenitor cells, the TATA box is replaced by TATAAT and is called the Pribnow box.
(ii) Transcription factors, promoters and RNA polymerase enzymes associate to form transcription complexes.
(iii) RNA polymerase enzyme opens up patches (at least 20 pairs) of DNA molecules and breaks hydrogen bonds. As a result, two single formulas are created. One is the template formula and the other is the coding formula.
(iv) Template formula acts as a template for making mRNA. It has two dots. start site and terminal site. Transcription starts from the start point.
2. mRNA Elongation
(i) RNA polymerase enzyme adds new nucleotides to the template. Nucleotides tend to join 3-5 carbons.
(ii) Uracil (U) is added to complement adenine (A) and cytosine (C) to complement guanine (G) according to base pairing principle. That is, if the ATTCGA sequence is present in the DNA source, the UAAGCU sequence is formed in the mRNA source. The portion of DNA molecule from which mRNA is produced is called transcription unit.
(iii) The mRNA chain continues to grow until the termination point of the template strand is reached. At the same time, the transcribed segment becomes double helix.
3. mRNA Termination: The mRNA sequence grows and touches the terminator point of the DNA. Touching the termination point releases the RNA polymerase enzyme. As a result, mRNA production stops. The mRNA thus produced is called pre-mRNA.
4. mRNA termination
(i) Capping: 7 guanosine nucleotides are added to the 5 end of pre-mRNA to form a guanine cap. This is called capping. It is also called 5 caps. It ensures ribosome attachment during translation.
(ii) Tailing: 50-150 adenines are added to the 3 end of pre-mRNA to form a poly A tail. This process is called tailing. It protects mRNA from the damaging effects of hydrolytic enzymes and accelerates transport.
(iii) Splicing: Pre-mRNA consists of two parts. Exon or coding part and Intron or non-coding part. Translation never occurs from an intron. Therefore, introns are excised from pre-mRNA during splicing process in the presence of spliceosome. The resulting final mRNA is obtained. The final mRNA is suitable for the cytoplasmic environment. therefore passes through the pores of the nucleus into the cytoplasm.